1. Field of the Invention
The present invention relates generally to a seal construction for a fluid swivel joint, and more particularly, pertains to a seal construction for a large diameter, high pressure fluid and gas swivel joint, such as is commonly used on offshore loading terminals for oil and gas tankers.
2. Description of the Prior Art
The development of subsea petroleum and gas production systems has included a concept wherein a floating processing and storage vessel incorporates as a significant component thereof a high pressure multiple passage fluid swivel joint. The fluid swivel joint essentially includes therein a plurality of stationary fluid passages or pipes extending downwardly from the interior of the fluid swivel joint to carry gas, oil, air, water, or other fluids either up thereto or down therefrom. A plurality of couplings are mounted on the exterior of the fluid swivel joint and are rotatable relative thereto, with each rotatable coupling communicating through the fluid swivel joint with one of the stationary fluid passages or pipes. Griebe U.S. Pat. No. 2,894,268 and Briatiani U.S. Pat. No. 3,590,407 disclose float supported sea terminals utilizing a concept very similar to that described above.
A problem associated with such a fluid swivel joint is in the provision of seals which will withstand the relatively high pressure differentials, often up to 6000 psi thereacross, while also providing for the relative rotational movements associated with the swivel joint. The seals which have been utilized in many of these fluid swivel joints have been V-type lip seals, which often resulted in very large frictional forces in the swivel joints when they are designed to accommodate the high pressure differentials.
Additionally, the high pressure differentials have also resulted in extrusion of the relatively pliant sealing members into the gaps which they are designed to seal. Prior art swivel joints have often been constructed with an outer ring which rotates about an inner ring with radial seals therebetween. High internal pressures often result in deflections in the rings which increase the radial clearance gap. Common seal materials cannot effectively bridge this gap while maintaining the pressure integrity of the sea. Therefore, common seal materials have been reinforced with various elements to increase the materials' physical strength in order to bridge the radial clearance gaps. Unfortunately, the reinforcing elements tend to render the seal material compounds less resilient and more abrasive. Consequently, a seal formed of these compounds must have a higher contact force in order to effect a seal because the less resilient materials have increased resistance to the filling into microgrooves in the mating seal surface so as to seal against any fluid passage. The higher contact stress and the more abrasive nature of the reinforcing elements of the material compounds tend to increase wear of both the seal and the seal mating surface, frequently leading to loss of pressure integrity of the seal. In order to solve the problems of extrusion of the seal materials into the gap, the prior art has also utilized antiextrusion rings of hard plastic or metal to support sealing elements and bridge the clearance gap between the rings of a fluid swivel joint. Unfortunately, these prior art designs have not proven to be totally satisfactory.